1. Field of the Invention
The present invention relates to an optical disc device for performing reproduction from and/or recording on an optical disc. The invention also relates to an optical disc determination method of determining a type of an optical disc.
2. Description of Related Art
Optical discs such as a compact disc (hereinafter referred to as CD) and a digital versatile disc (hereinafter referred to as DVD) have been in widespread use. Moreover, in recent years, in order to increase the information volume of an optical disc, studies on high-density recording on an optical disc have been advanced, and optical discs, such as a blue-ray disc (hereinafter referred to as BD), that are capable of recording a large volume of information have been put into practical use.
Information recording and reproduction on and from an optical disc are performed in an optical disc device. Since there are a plurality of types of optical discs as described above, in view of convenience, etc., optical disc devices capable of performing alone information recording and reproduction on and from a plurality of types of optical discs have become widespread. Typically, in such optical disc devices, when an optical disc is inserted into the device, a type of the optical disc is first determined, the device is set in condition in accordance with a corresponding optical disc based on a determination result, and then recording and reproduction are performed on and from the optical disc.
A large number of methods of determining a type of an optical disc in an optical disc device capable of performing information recording and reproduction on and from a plurality of optical discs have been conventionally reported. For example, Patent Document 1 suggests a method of determining a type of an optical disc by determining the recording density of the optical disc based on an RF signal. In addition, Patent Document 2 suggests another method of determining a type of an optical disc by generating a focus signal based on an RF signal.
As a method different from such methods of determining a type of an optical disc by using an RF signal, for example, an optical disc determination method as indicated in Patent Document 3 has been suggested. In the optical disc determination method of Patent Document 3, first based on a pull-in signal, a time length from detection of surface reflection on the optical disc surface to detection of recording layer reflection on a recording layer is measured to roughly classify a type of an optical disc. For example, between a CD and a DVD, due to a difference in the thickness of a transparent cover layer (for example, 1.2 mm for the CD and 0.6 mm for the DVD), the time length from the detection of the surface reflection to the detection of the recording layer reflection differs. Therefore, the measurement of this time length permits roughly classifying the type of the optical disc as a CD-type or a DVD-type. Then after the type of the optical disc is roughly classified, a peak value of a focus error signal is measured, and based on this peak value, detailed determination of the optical disc (for example, determination on whether a CD-type optical disc is a CD-ROM, a CD-R, or a CD-RW) is performed.
Patent Document 3 suggests a technique intended to reduce the time required for disc type determination processing upon determination of the type of the optical disc with such a method.
As described above, various optical disc determination methods have been conventionally suggested. For the purpose of improved accuracy in determining a type of an optical disc, the inventor considers roughly classifying a type of the an optical disc by using a pull-in signal (PI signal) as a sum signal of reflected light and then determining a type of the optical disc by using amplitude of the PI signal in addition to amplitude of a focus error signal (FE signal).
Different types of optical discs are designed to have different reflectance. Detecting the amplitude of the PI signal as a sum signal of reflected light permits determining the type of the optical disc.
However, in determining the type of the optical disc by use of the amplitude of the PI signal, the type of the optical disc is erroneously determined in some cases particularly for an optical disc with low reflectance. Discs including a BD pigment change type (BD-R) and a BD phase change type (BD-RE) have low reflectance, and thus this erroneous determination is likely to occur in such discs, which is problematic.
Review by the inventor in this regard has found the following points. When an objective lens is moved in, for example, a direction approaching the optical disc to determine a type of an optical disc, as shown in FIGS. 9A and 9B, a PI signal is obtained which indicates a peak at a zero crossing position of an S-shaped curve of an FE signal. FIGS. 9A and 9B are illustrative diagrams illustrating a signal obtained when the objective lens is moved in the direction approaching the optical disc. FIG. 9A shows the PI signal, and FIG. 9B shows the FE signal.
Observation of this PI signal showed that, when information is recorded on the optical disc, for example, a noise component as shown in FIG. 10 is generated near the peak of the PI signal. This proves that, since the reflectance on a recording layer of the optical disc changes when information is recorded onto the optical disc, as is the case with the PI signal as the sum signal of reflected light, an RF signal generated from the sum signal of reflected light is superimposed on the PI signal, which causes a noise.
It is assumed that when the RF signal is superimposed on the PI signal as described above, the type of the optical disc is erroneously determined for following reasons. Specifically, the amplitude of the PI signal is detected after the PI signal is subjected to A/D conversion and then loaded into a processor such as a CPU; therefore, the PI signal may be erroneously recognized as, for example, a signal as shown in FIG. 11 depending on sampling condition of the PI signal. As a result, the amplitude of the PI signal is judged to be low, resulting in erroneous optical disc determination. Then this erroneous determination is likely to occur particularly in an optical disc with low reflectance.
Conventionally, it has never been reported that there is a problem as described above involved in determining a type of an optical disc by use of a PI signal and also a way of resolving this problem has never been reported.                [Patent Document 1] JP-A-2005-353142        [Patent Document 2] JP-A-2004-206765        [Patent Document 3] JP-A-2005-259252        